Inductively coupled plasma (ICP) reactors use an inductive circuit element that is placed adjacent to, or inside a discharge region in order to couple energy from an RF power source to an ionizable gas. The inductive circuit element is typically a helical or spiral-like antenna structure. In certain plasma reactor systems, additional electrical reactances are used to tune the inductor, such that an electrical resonance occurs at the RF driving frequency.
When an inductive coupling element is driven in the electrical resonance condition, high potentials tend to exist on the structure which lead to a capacitive coupling to the plasma discharge. Capacitively coupled plasmas are characterized by high voltages which cause ions to be accelerated across the plasma sheath to the walls, at high energy, and can cause sputtering and heating of the walls. The prior art has reduced capacitive coupling into plasma sheaths by utilization of encompassing Faraday shields that are slotted so as to enable penetration of the inductively created energy, while shorting out the capacitive coupling.
J. Hopwood in "Review of Inductively Coupled Plasmas for Plasma Processing", Plasma Sources Science & Technology, Volume 1, 1992, pages 109-116, provides a review of a number of plasma reactor structures that are used for ICP processing. Several reactor versions employ helical inductors that are wound around a plasma chamber and are separated therefrom by slotted Faraday shields. A further version uses a planar spiral coil which is placed on a quartz window and induces energy into the plasma chamber through the quartz window.
J. Keller in "Inductive Plasmas for Plasma Processing", Plasma Sources Science & Technology, Volume 5, 1996, pages 166-172, provides further details regarding ICP reactor structures. Keller indicates that non-uniformities present in inductively induced plasmas can be reduced by increasing the turn height or width of the inductor that is used to create the plasma. Further, Keller indicates that the distance between coil turns in the middle of the coil, or where the plasma density is at a maximum, can also be increased. He further indicates that the RF fields can be reduced by an increase in the thickness of the dielectric window through which the energy is coupled. Also, he indicates that the plasma can be radially confined by multi-pole magnetic cusps and that gas injection can be improved through use of a "showerhead" so as to reduce non-uniformities in the resulting plasma sheath.
U.S. Pat. No. 5,556,521 to the Applicant herein, describes a sputter etching apparatus wherein an inductor is placed on a dielectric plate and induces a plasma within a chamber positioned beneath the dielectric plate. A contoured pocket in the dielectric plate receives the inductor and enables the creation of a high density, uniform plasma proximate to a substrate located in the plasma chamber.
ICP reactors are utilized for deposition and etching of materials, principally in the semiconductor industry. For example, plasma enhanced chemical vapor deposition (PECVD) of dielectrics and metal thin films onto large area wafers comprise standard manufacturing processes in the semiconductor industry. Further, such reactors are employed for dry etching of dielectric and metal thin films. ICP reactors are also used for manufacture of thin film transistors on large area glass substrates (e.g., for use in flat panel displays). Likewise, such reactors are used to create patterned dielectric and metal thin films on large area glass substrates.
In all such applications, it is desirable that capacitive coupling between the inductor and the plasma be minimized. As the plasma is most intense in the vicinity of the inductor assembly, there is a large ion flux directed toward the inductor. These ions are accelerated by a large sheath potential and sputter the underlying surface of the dielectric plate or window through which the inductive energy is coupled. Such sputtering forms particulates which contaminate a wafer or substrate being processed.
Because current ICP reactors operate at low pressures, the dielectric window through which the inductive energy is coupled is required to exhibit a substantial thickness so as to resist fracture due to the differential pressure that is present thereacross.
It is therefore an object of this invention to provide an improved ICP reactor wherein the structure of the inductor is arranged to enable a field configuration to be induced in the reactor chamber so as to provide a substantially uniform plasma therein.
It is another object of this invention to provide an ICP reactor which is enabled to utilize a dielectric window of relatively thin cross-section that reduces attenuation seen by energy that is inductively coupled therethrough.
It is yet another object of this invention to provide an improved ICP reactor wherein an incorporated Faraday shield substantially reduces capacitive coupling into the reactor, provides gas distribution and dielectric window support.